Experimental work on the subarachnoid space will be continued with examination and analysis of results by both transmission and scanning electron microscopy. The responses of free cells in the subarachnoid space will be observed after challenging this area with foreign proteins (horseradish peroxidase, selected antigens, etc.). Injection into the cisterna magna initiates a pronounced disease fighting reaction in 2 to 4 hours. This involves ingestion of foreign material, clumping of macrophages, and other phenomena. These reactions will be observed in considerable detail in response to various types of foreign protein.